A useful approach to understanding the way in which muscles work is to study the cross-bridge properties in considerable detail. In this regard, force-velocity relations and contraction transients, in intact muscles as well as in skinned fiber preparations, have provided important physiological information about the cross-bridge mechanisms. The present study proposes to evaluate the cross-bridge kinetics in frog and mammalian skinned fibers in particular with regard to the action of calcium. This will be done by making the physiological measurements in these fibers at varying calcium concentrations at different points along the length-tension relation as well as at different values of ionic strength. The results are expected to give: 1) a better understanding of the mechanisms of regulation of contraction properties of the skeletal muscle by calcium; 2) a more complete and clearer understanding of the molecular principles underlying the length-tension diagrams; and, 3) enhance knowledge regarding the way in which a biochemical parameter, ionic strength, affects the cross-bridge properties instructurally intact preparations. Since one of the crucial abnormalities in the major types of muscular dystrophies and myopathies is in the contractile machinery, the present techniques and results will help in ultimately understanding of the nature of the disease. The approach to be developed here can eventually be used on the diseased muscles in order to specify the molecular basis for the different types of dystrophies and myopathies.